a Kamerlingh Onnes Laboratorium, Leiden University, PO Box 9506, 2300, RA Leiden, Netherlands
b Natuurkundig Laboratorium, Universiteit van Amsterdam, PO Box 20215, 1000, HE Amsterdam, Netherlands
Abstract:
Specific heat data below 1 K of Bi2Sr2CaCu2O8 and YBa2Cu3O7 are analyzed. For YBa2Cu3O7 the nuclear specific heat, CN, amounts to 38T−2 μJ/mol K. CN for Bi2Sr2CaCu2O8 exceeds that of YBa2Cu3O7 by a factor of 15. The nuclear quadrupolar specific heat contribution alone is insufficient to explain the data for YBa2Cu3O7, while lack of NQR data does not allow such a comparison in Bi2Sr2CaCu2O8 to be made. The contribution to CN from nuclear spins coupled via the contact hyperfine interaction with correlated magnetic spins (in the CuO2 plane) is derived as a function of the correlation length. This contribution can be treated independently from the quadrupolar term. We show that the excess specific heat in YBa2Cu3O7 likely originates in a few percentage of an impurity (oxygen deficient) phase with a strong hyperfine field on the Cu nuclei.